A downhole acoustic logging tool generally contains one or a plurality of acoustic sources and an array of acoustic receivers. The one or a plurality of acoustic sources can be of a multi-pole nature (generally monopole, dipole and quadrupole). A multi-pole source is made up of two more elements placed at the same axial location on the tool and generally equally spaced around the circumference. At least two source elements are needed for an acoustic dipole field and at least four are needed for a quadrupole field. The array of acoustic receivers is decided so as to capture the multi-pole nature of the propagating acoustic field. In general 4-12 acoustic receivers are placed along the tool axis; with each receiver comprising one or more acoustic sensing elements distributed along the tool circumference (at least two are needed to capture dipole and at least four are needed to capture quadrupole).
A logging event consists of firing the one or more individual elements of an acoustic source to generate a desired acoustic field in the borehole (generally monopole, dipole or quadrupole); and recording the acoustic (pressure) signal at each of the receivers' sensing elements, as it propagates along the wellbore. The recorded traces are processed, first to extract the different acoustic field components (monopole, dipole or quadrupole). Each of those field components is further processed to extract acoustic properties of the formation being traversed by the wellbore.
It is widely known that in acoustic logging while drilling, because of the generally strong acoustic noise generated by the drilling operation, there is a need to stack (add up) the traces recorded by several individual logging events (typically 16-64 traces) in order to enhance the signal to noise ratio. The basic idea is that the signal being excited by repeated firings of the acoustic source will be nicely correlated and therefore will add up. In contrast, the signal from a drilling noise will be largely uncorrelated and therefore will tend to cancel out. The stacking then improves the quality of the subsequent processing.
Stacking works well when the tool is not rotating between source firings or the formation being traversed by the wellbore exhibits acoustic isotropic behavior around the wellbore axis i.e. if the acoustic field generated in the wellbore by the firing of a multi-pole source is the same independent of the orientation of the source as the tool rotates. However, more often than not, this is not the case, particularly when drilling highly deviated wells. In this situation, the stacking will tend to average out important information about the anisotropic properties of the formation.
It is possible to have sensors in the tool that measure and record the instantaneous tool face angle (i.e. the instantaneous orientation as the tool rotates) that goes along with each logging event. The stacking can then be done on logging events that correspond to similar (or equivalent) instantaneous tool orientations. However, the stacked events will generally extend over an appreciable number of tool revolutions, and the tool might have moved appreciably (because of the drilling operation) during that time. The stacking will tend to average out variations in the rock properties along the wellbore.
Therefore, a better solution is desired to improve the quality of acoustic logging while drilling when the formation exhibits acoustic anisotropy.